This invention relates to light collection systems, and more particularly relates to a light collection system for the collection of light from an intense light source such as an arc lamp, which light is to be employed in a projection display system.
Projection display systems employ intense light sources in order to provide sufficient light for a bright display at the viewing screen after modulation of the light by one or more light valves. Light sources widely used for this purpose include arc lamps such as UHP and xenon lamps. The elongated shape of the arc of such lamps presents a challenge to the designer to provide efficient optical systems for the collection and shaping of the emitted light for subsequent modulation and display.
The most common light collection method in projection is based on a parabolic or elliptical reflector and a lenticular array to correct for non-uniformity of the light source. Due to the large aberrations typical of these reflector types and the mismatch between light source and lenticular geometry, etendue (angular extent of the beam) is not preserved. This accounts for the poor light utilization of present projectors, especially in the case of small light valves.
U.S. Pat. No. 6,231,199, issued to Li on May 15, 2001, teaches an optical system for collecting and condensing light from one or more arc lamps down to a small spot size for coupling to a target such as the input face of a single optical fiber. The system includes a retro-reflector for effectively doubling the light output of the arc lamp, and a plurality of concave paraboloid reflectors.
An exemplary arrangement using two parabolic reflectors back-to-back to create an arc image at near unit magnification is shown in Applicant""s FIGS. 1A through 1C.
FIG. 1A is a longitudinal section view of a light collection system 10 for an arc lamp light source 12 having an elongated arc 14. This view corresponds to the axial image plane of arc 14. The light collection system includes retro-reflector 16 and a compound parabolic reflector 18, composed of primary and secondary parabolic reflector sections 18a and 18b. Light from the lamp represented by rays R1 and R2 is retro-reflected back onto the arc 14 by retro-reflector 16. Parabolic reflector sections 18a and 18b are coaxial, and arc 14 is located at the focus of the primary parabolic reflector section 18a. Light from arc 14, represented by rays R3 and R4, thus forms arc image 20 at the focus of the secondary parabolic reflector 18b, where an entrance face 22a of optic fiber 22 is positioned.
Because of the symmetry of this arrangement the huge aberrations of a parabola are largely cancelled. The cone angles of the light emitted from the arc are determined by the lamp""s radiation characteristic. The cone angles of the present UHP lamp are very large. As shown in FIGS. 1B and 1C, the cone angle "psgr" in the axial image plane, and the cone angle xcfx80 in the radial plane (view AAxe2x80x2), are about 90 degrees and 180 degrees, respectively, producing an anamorphic cone. Cone angles this large can not be handled downstream by conventional imaging optics. Non-imaging shaping means, e.g., a parabolic reflector, would re-introduce the parabolic aberrations, defeating the purpose of the concept. Thus, although the arrangement of FIG. 1 makes the arc image 20 accessible, it does not solve the problem of how to carry the light from there efficiently and in a manner which preserves etendue.
In accordance with the invention, the secondary parabolic section is split into segments and each segment is shifted either longitudinally (along the arc axis) or radially (about the arc axis), causing the formation of multiple arc images (one for each segment) which are correspondingly shifted with to the arc axis.
The light cone associated with each segment can be made arbitrarily small such that each arc image can be accommodated by conventional optics. Preferably, however, each arc image from the light collection system is directly coupled into a loss-less, etendue-preserving light guide of the type described in co-pending U.S. patent application Ser. No. 10/161,798, filed Jun. 4, 2002 assigned to the present Assignee
By providing a separate light guide input section for each arc image having an input face sized to fit the arc image and then inputting the images separately) into a light guide body section having a common input a sized to fit the array of arc images, aperture shaping can be achieved, for instance, concatenating the separate images by aligning them end-to-end to match the stripe geometry required for a single panel scrolling color projector.
A xe2x80x9cfittingxe2x80x9d light guide is one which has an input face sized to fit a single arc image or an array of arc images which are adjoining or partially overlapping, but not completely superimposed.
According to one aspect of the invention, a light collection system comprises a parabolic reflector, the parabolic reflector comprising a primary parabolic reflector section and a secondary parabolic reflector section, the sections being paraboloid sections and being positioned coaxially with respect to one another, so that an object placed at the focal point of the primary parabolic reflector section is imaged at the focal point of the secondary parabolic reflector section,
characterized in that the secondary parabolic reflector section is divided into a plurality of segments, whereby for each segment, a separate image is produced.
According to another aspect of the invention, a light engine for a projection display system is provided, the light engine comprising the light collection system of the invention, and a loss-less, etendue-preserving light pipe having a plurality of input faces and an output face, each input face positioned to input one of the plurality of images produced by the light collection system.
According to a further aspect of the invention, a projection display system is provided, the system comprising the light engine of the invention, at least one light valve for modulating light from the light engine to produce a display in accordance with a display signal, and a projection lens for projecting the display onto a display surface.
The light collection system of the invention provides low aberration reflective optics that can be replicated at low cost, and enables high-efficiency light collection and aperture shaping without loss of etendue. This opens the way for high-efficiency projection with small size light valves. Particularly, single panel scrolling color systems using narrow stripe illumination will benefit from this invention.